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Applied Catalysis A: General 337 (2008) 58–65

Acidity and defect sites in titanium silicalite catalyst

Gang Yang a,b,*, Xijie Lan a, Jianqin Zhuang a, Ding Ma a, Lijun Zhou b,Xianchun Liu a, Xiuwen Han a, Xinhe Bao a,*

a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR Chinab Key Laboratory of Forest Plant Ecology, Northeast Forestry University, Ministry of Education, Harbin 150040, PR China

Received 20 June 2007; received in revised form 12 November 2007; accepted 22 November 2007

Available online 3 December 2007

Abstract

With density functional calculations and in situ 29Si and 31P MAS NMR experiments, a novel defect site (defect site II, dsII) was identified in

titanium silicalite molecular sieve (TS-1 zeolite), supported by the previous experimental results. The conversion from Q3 species [(SiO)3Si(OH)]

into Q4-like species [Si(OSi)4] due to TMP adsorption indicated the presence of Bronsted acidity in TS-1 zeolite. The thermodynamic analysis and

Ti/Si substitution energies further confirmed that the proton transferring process of defect site II is much facilitated in TS-1 zeolite whereas on the

contrary in silicalite-1. With the proton-affinity calculations, the Bronsted acidities of several cation-doped zeolites were found to decrease in the

order of Al-ZSM-5 > Fe-ZSM-5 > B-ZSM-5 > TS-1. The Bronsted acidity of TS-1 zeolite is even weaker than that of B-ZSM-5 zeolite,

suggesting the potential role as a new acid catalyst. As to silicalite-1, its acidity is even weaker than that of defect site I (dsI) in TS-1 zeolite.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Acidity; Defect sites; Density functional calculations; MAS NMR; Proton transfer

1. Introduction

Titanium silicalite molecular sieve (TS-1 zeolite) was

discovered in 1983 by Taramasso et al. [1], and since then it has

gradually been acknowledged as one milestone in hetero-

geneous catalysis. With hydrogen peroxide (H2O2) as the

oxidant, the TS-1 zeolite shows excellent performances to

many types of selective oxidation processes such as hydro-

carbons, alkenes and, etc. [2–4].

Generally, the titanium species in the micro- and meso-

porous materials were categorized into two types: framework Ti

atoms substituted into the silica lattices [3–16] and extra-

framework Ti atoms grafted on the surfaces of carriers [17,18].

On the well-prepared TS-1 samples, the Ti atoms were found to

be incorporated into the lattice as the isolated sites and

surrounded by four OSiO3 tetrahedra, see the so-called perfect

sites (ps) in Scheme 1. With the aid of XRD experiments,

* Corresponding authors at: State Key Laboratory of Catalysis, Dalian

Institute of Chemical Physics, Chinese Academy of Sciences, Dalian

116023, PR China. Tel.: +86 411 84379528; fax: +86 411 84694447.

E-mail addresses: dicpyanggang@yahoo.com.cn (G. Yang),

baoxh@dicp.ac.cn (X. Bao).

0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2007.11.037

Millini et al. [19] observed that the unit cells of TS-1 zeolites

will increase linearly as the loadings of framework Ti atoms are

increased. Through the analysis of IR, Raman and X-ray

absorption near-edge spectroscopy (XANES) results, Ric-

chiardi et al. [20] considered that the peaks of IR and Raman

bands at 960 and 1125 cm�1 as well as the peak of XANES

spectra at 4967 eV were quantitatively associated with the

tetrahedral Ti sites. In addition, the extended X-ray absorption

fine structure (EXAFS) technique observed that the local

geometries of the framework Ti atoms undergo symmetric

changes from the Td type to the Oh type when the probe

molecules such as water (H2O) and ammonia (NH3) are

adsorbed on the TS-1 zeolites [21]. It indicates that the

framework Ti species display Lewis acidic properties towards

these probe molecules, and such a viewpoint was confirmed by

subsequent theoretical results of Sinclair et al. [5], Damin et al.

[10] and Bordiga et al. [22]. Besides the perfect sites, the Ti–O–

Si bridges in TS-1 zeolite can be hydrolyzed and form the

titanol (Ti–OH) and silanol (Si–OH) pairs [5,20,23], analogous

to the ruptures of Si–O–Si bridges in Al-incorporated zeolites.

This defect site was denoted here as defect site I (dsI), see

Scheme 1. Interestingly, recent density functional calculations

revealed that the Ti sites adjacent to the Si vacancies rather than

Scheme 1. Two types of framework titanium species previously observed in

TS-1 zeolite. (a) Perfect site; (b) defect site I (dsI) hydrolyzed from perfect site.

G. Yang et al. / Applied Catalysis A: General 337 (2008) 58–65 59

those of the perfect sites are of higher reactivity for the reaction

of propylene epoxidation [24]. Using in situ MAS NMR

technique, Zhuang et al. [14] studied the oxidation reaction of

trimethyphosphine (TMP) into trimethylphosphine oxide

(TMPO) and found that this reaction was catalyzed by two

types of Lewis acids. The first Lewis acid is related with the

perfect site whereas the second related with defect site I (dsI)

[14,25].

Another 31P MAS NMR peak centered at �4.8 ppm was

observed in the previous 31P MAS NMR experiments [14],

which is unresolved up to date. On the basis of the results of Al-

incorporated zeolites, this peak should be due to the formation

of the ionic [HP(CH)3]+ species [26]. It is an indication that

there should be Bronsted acidic sites in TS-1 zeolite. However,

no reports have yet been given as to the Bronsted acidic sites in

TS-1 zeolite, let alone their potential applications in catalytic

processes. In the present work, multi-experimental techniques

such as XRD, UV–vis and MAS NMR were employed to

resolve the Bronsted acidic sites of TS-1 zeolite. Density

functional theoretical methods were also used to understand the

acidic sites and defect sites in titanium-related materials.

2. Experimental

2.1. Synthesis and characterization of TS-1 zeolite

The TS-1 samples with different Si/Ti ratios were kindly

provided by BASF Corporation, which were synthesized using

the preparation methods as reported elsewhere [27,28]. The

elemental analysis was completed with n Magix 601 X-ray

fluorescence (XRF) spectrometer. The structures of the TS-1

samples were checked with a D/max-gb type X-ray diffract-

ometer (Rigaku) using monochromatic Cu Ka radiation (30 kV

and 15 mA), with scan speed of 58/min in 2u. After being

pretreated at the temperature of 813 K for 5 h, the TS-1 samples

were submitted under UV–vis spectra, which were recorded on

a JASCO V-550 spectrometer with scan range of 190–490 nm.

2.2. NMR procedure

With a special device [29], the TS-1 samples were heated

and dehydrated at 673 K in vacuum (below 10�2 Pa) for about

20 h. Then by being exposed to the saturated TMP vapor at

room temperature (from Acros Organics), the samples were

adsorbed with TMP for 30 min. In order to remove the

physisorbed TMP molecules as complete as possible, the TS-1

samples were evacuated for nearly 30 min. After such

pretreatments, the TS-1 samples were filled in situ into the

NMR rotors without exposure to air. The NMR experiments

were carried out on a Varian Infinityplus-400 spectrometer

equipped with a 4.0 mm MAS probe. The 29Si MAS NMR

spectra with high-power proton decoupling were recorded at

79.4 MHz with 2048 scans, 2.8 ms pulse delay and at 6 kHz

spinning rate. The 31P MAS NMR spectra with high-power

proton decoupling were recorded at 161.8 MHz, using a 2.0 ms

pulse, 4 s repetition time and 2048 scans and at 10 kHz spinning

rate. The 29Si and 31P NMR chemical shifts were referenced to

4,4-dimethy-4-silapentane sulfonate sodium (DDS) and 85%

H3PO4, respectively.

3. Calculational details

All density functional theory (DFT) calculations were

performed under Gaussian98 program [30]. Hybrid B3LYP

exchange correlation functional [31,32] was employed, for it

provides good description of reaction profiles, particularly

geometries and reaction heats. The commonly used 6-31G*

basis was applied to all the elements except Ti and Fe. To

describe Ti and Fe, the core electrons were represented by

LANL2DZ effective core potential (ECP) [33–35], with the

valence electrons by LANL2DZ basis as treated in our previous

work [15,16,25].

According to the results of neutron power diffraction

experiments [7,8], the T7 site was the most probable location

for the Ti atoms substituted into MFI-type zeolite. Therefore,

the Ti atoms in all the clusters occupied the T7 sites. As Fig. 1

shows, the cluster models contain 17 T sites, which are much

larger than the usually adopted five T ones [36,37]. The

boundary Si atoms were saturated with H atoms with the Si–H

distances fixed at 1.500 A´

.

To further understand the proton transfer processes in defect

sites of TS-1 zeolite, cluster models were selected from

silicalite-1, with the TMP adsorption behaviors also studied. To

further understand the Bronsted acidity in TS-1 zeolite as well

as its strength, the Bronsted acidities of three other cation-

doped ZSM-5 zeolites (cations = B, Al and Fe) were under

investigation and compared with that of TS-1 zeolite. It should

be noted that the cluster sizes of silicalite-1, B-, Al- and Fe-

ZSM-5 zeolites are exactly the same with those of TS-1 zeolite

and optimized under the same conditions.

4. Results and discussion

4.1. Characterization of TS-1 zeolite

Through the elemental analysis by XRF spectrometer, it was

found that the three TS-1 samples have Si/Ti ratios of 41.2, 51.9

and 56.9, respectively. Moreover, only three elements of Si, Ti

and O are present in the TS-1 zeolites; i.e., metal ions of +III

valence states such as B, Al and Fe were excluded.

Fig. 1. Two interacting modes between defect site II in TS-1 zeolite and the probe molecule TMP (O1 and O2 atoms are the bridging hydroxyls whereas O3–O5 and

O6–O8 atoms are bonded only with the Ti and Si atoms, respectively. The Si–O2 distance in (b) equals 2.758 A). (a) Covalent complex with TMP physisorbed on

defect site II; (b) ionic complex with the formation of [HP(CH)3]+ species.

G. Yang et al. / Applied Catalysis A: General 337 (2008) 58–6560

The XRD experiments showed that the TS-1 samples

possess MFI type of structure with good crystallinity (Fig. 2). A

strong transition centered at 210 nm appeared in the UV–vis

spectra (Fig. 3), which originates from the electronic transfer of

the p:–p: transitions between the framework titanium species

[38] and the oxygen atoms. It is characteristic of the isolated

tetrahedral-coordinated Ti4+ cations [39], indicating that no

extra-framework anatase is present in the TS-1 samples.

4.2. 31P and 29Si MAS NMR spectra

As indicated by the density functional results of Section 4.5,

the Bronsted acidity of TS-1 zeolite is very weak. The weak

Bronsted acidic sites cannot even be well resolved from the 1H

MAS NMR signals. The 31P MAS NMR spectra of the TS-1

samples adsorbed with TMP were recorded and shown in Fig. 4.

The TS-1 zeolites have different Si/Ti ratios of 41.2, 51.9 and

56.9, respectively. The peaks at �3.9 ppm is ascribed to the

formation of the ionic [(CH3)3P-H]+ complex [26], implying

the presence of Bronsted acidic sites in TS-1 zeolite. With

(NH4)2HPO4 as the exterior criterion, the contents of the

Bronsted acidic sites were obtained, which amount to 0.098,

0.101 and 0.109 mmol g�1 for TS-1 zeolites with the Ti/

Fig. 2. The XRD spectrum of TS-1 zeolite with Si/Ti ratio of 41.2.

(Ti + Si) ratios of 0.017 (Si/Ti = 56.9), 0.019 (Si/Ti = 51.9) and

0.024 (Si/Ti = 41.2), respectively. It can be found that the

amount of Bronsted acidic sites increase with the Ti content

(Fig. 5). It was also observed that the 31P MAS NMR peak to the

Bronsted acidic sites remains almost invariant at �3.9 ppm,

indicating that the interacting strength between TMP and TS-1

zeolite has no relationship with the Ti content.

Fig. 6 displays the 29Si MAS NMR spectra of TS-1 zeolites

before and after TMP adsorption. The 29Si MAS NMR peaks at

�113.5 and �103.5 ppm were assigned to the Q4 species

[Si(OSi)4] and the Q3 species [(HO)Si(OSi)3], respectively

[13,26]. Before TMP adsorption, the ratio of Q4/Q3 species

amounts to 12.3, which is increased to 19.8 due to the TMP

adsorption. It indicates that some new Q4-like sites will be

formed from the conversion of Q3 sites.

4.3. Theoretical studies on the proton transfer process

As aforementioned, there are only three elements of Si, O

and Ti present in the TS-1 samples, and all the Ti atoms were

Fig. 3. The UV–vis spectrum of TS-1 zeolite with Si/Ti ratio of 41.2.

Fig. 4. The 31P MAS NMR spectra of TS-1 zeolite adsorbed with TMP.

Fig. 5. The influence of the Ti/(Ti + Si) ratios on the contents of the Bronsted

acidic sites in TS-1 zeolite.

Scheme 2. The novel defect site II (dsII) built from the hydrolysis of the perfect

sites, where both of the two bridging OH groups are coordinated to the Ti atom

whereas only one of them coordinated to the Si atom (the dotted line indicates

weak interactions instead of direct bonds between Si and O atoms).

G. Yang et al. / Applied Catalysis A: General 337 (2008) 58–65 61

incorporated into the zeolite framework. It is well known that

there is no acidic proton around the perfect site (ps) and

therefore this site will not produce the Bronsted acidity. As to

defect site I (dsI), the proton of the titanol group (TiO–H) was

presumed to have been transferred to TMP in our model

designs; i.e., the geometry-optimizations were initiated with the

structure of �TiO� and the ionic [H(PCH3)3]+ species.

However, as the optimization process continues the proton

will be gradually shifted back to the TS-1 zeolite, and the final

optimization results showed that the proton has been returned to

Fig. 6. The 29Si MAS NMR spectra of TS-1 zeolite before (a) and after (b)

adsorption of TMP. The TS-1 sample has a Si/Ti ratio of 41.2.

the TS-1 cluster. Accordingly, defect site I (dsI) has no Bronsted

acidity, either. The Bronsted acidic sites in TS-1 zeolite [13–15]

should result from a novel defect site present in TS-1 zeolite,

which was successfully resolved through the persistent

modeling designs, see defect site II (dsII) in Scheme 2. This

novel defect site is closely correlated with both titanol and

silanol groups, in good consistency with the previous and

present experimental results [13]. In Fig. 1(a), the TMP

molecule was physisorbed on defect site II and forms the

covalent complex. The optimized (TiO–)H–P distance is

equivalent to 2.242 A´

, suggesting that hydrogen bonding was

formed between defect site II and the adsorbed TMP molecule.

There are five short Ti–O distances with their exact values

obtained at 1.798, 1.804, 1.816, 1.858 and 2.247 A´

, respectively

(see Table 1). Accordingly, the Ti atom in defect site II is

directly associated with five neighboring O atoms. For the Si

atom in defect site II, it has also five neighboring O atoms.

However, the Si–O2 distance was optimized to be 2.758 A´

and it

is much beyond the tolerance of direct Si–O bonds.

Accordingly, the Si atom was considered to have formed

direct bonds with three –OSi and one –OH groups. It is

characteristic of the Q3 species [(SiO)3Si(OH)] in the 29Si MAS

NMR spectroscopy. The acidic proton in defect site II can be

Table 1

TMP adsorbed on defect site II (dsII) in TS-1 zeolite

Covalent complex Ionic complex

Ti–O (A´

) 1.798, 1.804, 1.816,

1.858 (O2), 2.247 (O1)

1.802, 1.855, 1.880,

1.902 (O2), 1.948 (O1)

Si–O (A´

) 1.626, 1.641, 1.647,

1.676 (O1), 2.758 (O2)

1.637, 1.642, 1.675,

1.629 (O1), 2.800 (O2)

O1–H1 (A´

) 2.242 1.702

Ti–Si (A´

) 3.433 3.257

n(Ti) 4.69 4.73

G. Yang et al. / Applied Catalysis A: General 337 (2008) 58–6562

transferred to the adsorbed TMP molecule and thus form the so-

called ionic complex [HP(CH3)3]+, see Fig. 1(b). That is to say,

the novel defect of dsII does display Bronsted acidity towards

the probe molecules (e.g., TMP in this study). In the ionic

complex, the Ti atom also forms direct bonds with five adjacent

O atoms as evidenced from the corresponding Ti–O distances,

see Table 1. However, the coordination environment of the Si

atom in the ionic complex has been changed, which is now

bonded with three –OSi and one –OTi groups. The coordination

environment of the Si atom should be ascribed as the Q4-like

species [Si(OSi)4] in the 29Si MAS NMR spectroscopy.

Therefore, the proton transfer process will bring about the

changes in the Si coordination environments from Q3 species to

Q4-like species, in fine agreement with the present 29Si MAS

NMR results.

Cluster models of the same sizes were taken from silicalite-1

and optimized with the same theoretical methods as those of

TS-1 zeolite. Analogous to TS-1 zeolite, it was found that the

perfect site and defect site I (dsI) of silicalite-1 do not show

Bronsted acidities. The latter proton-affinity results showed that

the acidity of defect site I in silicalite-1 (i.e., the silanol group)

is weaker than that of defect site I in TS-1 zeolite (i.e., the

titanol group). The covalent and ionic complexes of defect site

II (dsII) adsorbed with TMP were also obtained in sililciate-1, as

shown in Fig. 7. In the covalent complex, hydrogen bonding is

formed between defect site II and the TMP molecule with the

H–P distance located at 2.256 A´

. The Si atoms in defect site II

are four-coordinated, differentiating the five-coordinated Ti

atom in TS-1 zeolite. In the ionic complex, the Si atom at the T7

site is bonded with five neighboring O atoms, which is not often

found in silica-based materials. Accordingly, the proton

transfer process may not be favored in silicalite-1.

In TS-1 zeolite, the energy difference between the ionic and

covalent complexes was calculated to be 2.9 kcal mol�1. Such a

small energy barrier can be easily crossed over and therefore

the conversion from the covalent to the ionic complexes was

much facilitated in TS-1 zeolite. However, the same conversion

in silicalite-1 requires a much higher energy barrier of

22.4 kcal mol�1. It indicates that the proton transferring

process is difficult to take place in silicalite-1. Accordingly,

Fig. 7. Two interacting modes between defect site II of silicalite-1 and the probe mol

Si atoms are four-coordinated (covalent complex with TMP physisorbed on defec

coordinated to O2 and O6–O8 atoms (ionic complex with the formation of [HP(CH

TS-1 zeolite shows Bronsted acidity whereas silicalite-1 does

not. Besides the high energy barrier of proton transfer, the

absence of Bronsted acidity in silicalite-1 was supported by

several other proofs: (1) the previous and present NMR

experiments showed that the Bronsted acidic sites are closely

related with both titanol and silanol groups [13], and the present

NMR results further pointed out that the amount of Bronsted

acidic sites is dependent on the content of the framework Ti

species in TS-1 zeolite; (2) the proton affinities (PAs) of the

defect sites in silicalite-1 were calculated (see the details in

Section 4.5), and the results showed that the acidity of defect

site II in silicalite-1 is even slightly weaker than that of defect

site I in TS-1 zeolite, which is well acknowledged to have no

Bronsted acidity; (3) the adsorption of 15N enriched 2-

butanone[15N]oxime on sililicalite-1 was previously performed

by Parker [40], and only the signals of physisorption were

observed in their 15N NMR experiments.

As mentioned above, the expansions of the coordination

numbers in the T7 sites will be accompanied during the proton

transfer processes. The Ti atom other than the Si atom was

preferred as to the expansion of coordinations, which can be

well elucidated with the substitution energies. The substitution

energies of the Si atoms by the Ti atoms into MFI-type zeolite

(Esub) were estimated with the following equation:

silicalite-1 ! TS-1 (1)

Esub¼EðTS-1ÞþEðSi4þÞ�Eðsililcate-1Þ�EðTi4þÞ (2)

where E(TS-1), E(sililcate-1), E(Si4+) and E(Ti4+) represent the

energies of TS-1 zeolite, sililcate-1, Si4+ and Ti4+ ions, respec-

tively.

With respect to defect site II (dsII) (see Scheme 2), the acidic

proton can be deprived of and form the corresponding anion,

which resembles much the local structure of TS-1 zeolite in the

ionic complex containing the [HP(CH3)3]+ species. Before the

deprivation of the acidic proton, there is one neutral structure in

TS-1 zeolite and silicalite-1, respectively, with the Ti/Si

substitution energy denoted as Esub(neutral). After the

deprivation of the acidic proton, there is an anion in TS-1

and silicalite-1, respectively, with the Ti/Si substitution energy

ecule TMP [O1 and O2 atoms are the bridging hydroxyls. (a) Both of the marked

t site II); (b) one is five-coordinated to O1–O5 atoms whereas the other four-

)3]+ species)].

G. Yang et al. / Applied Catalysis A: General 337 (2008) 58–65 63

denoted as Esub(ionic). Accordingly, the difference in the Ti/Si

substitution energies after and before the acidic proton transfer

can be represented with the equation shown below:

DEsub ¼ EsubðionicÞ � EsubðneutralÞ (3)

The Esub(neutral) and Esub(ionic) were calculated to be 211.6

and 194.1 kcal mol�1, respectively. Accordingly, the DEsub

value is equal to �17.5 kcal mol�1. It indicates that the Ti/Si

substitution was much facilitated in the case of the anions

where the acidic protons have been transferred to the probe

molecules. That is to say, the proton transfer process will highly

stabilize the local structure of TS-1 zeolite instead of silicalite-

1. As a result of the proton transfer processes, the TS-1 zeolite

displays the Bronsted acidity towards the probe molecules. The

less preference of defect site II in silicalite-1 as anions is

probably caused by the appearance of five-coordination Si

atoms (Fig. 7(b)), and as a result silicalite-1 will not show

Bronsted acidity, consistent with the above thermodynamic

analysis and the previous 15N NMR experimental results [40].

The fact that silicalite-1 has no Bronsted acidity is also an

indication that the Bronsted acidic sites should be associated

with the Ti atoms, which agrees well with the present 31P MAS

NMR results that the content of Bronsted acidic sites increases

linearly with that of framework Ti atoms (Fig. 5). Moreover, in

TS-1 zeolite with high Si/Ti ratios, the two framework Ti atoms

were separated far away from each other, and it is very likely for

the Ti and Si atoms to cooperate and contribute to the formation

of Bronsted acidic sites as revealed by our previous NMR

experiments [13].

4.4. Coordination of the T7 atoms in defect site II

In defect site II (dsII), the coordination number of the Ti

atom with its first shell O atoms can be obtained with the

following equation:

nðTiÞ ¼ r0

r1

þ r0

r2

þ r0

r3

þ r0

r4

þ r0

r5

(4)

where r1, r2, r3, r4 and r5 are the five Ti–O distances in the

defect site, and r0 is the averaged Ti–O distance in the corre-

sponding perfect site. In the covalent complex, the Ti coordina-

tion number is calculated to be 4.69, and in the ionic complex it

amounts to 4.73. Accordingly, the Ti coordination number of

this novel defect site (dsII) should be approximately 4.7. In a

similar way as Eq. (4), the Ti coordination number of defect site

I (dsI) was obtained to be 3.98. Within theoretical errors, the Ti

coordination numbers of perfect sites and defect site I (dsI) can

be considered as four. Therefore, the actual Ti coordination

number with its first O shell atoms should fall within the range

of 4.0–4.7 in TS-1 zeolite. Lamberti et al. [41] carried out the

EXAFS and photoluminescence experiments on the TS-1

zeolite, finding that the first shell Ti coordination number is

higher than four and within the range of 4.44 � 0.25. The

perfect agreement on the Ti coordination with the previous

EXAFS results provides a powerful support to our density

functional theoretical results on defect site II (dsII).

4.5. Strength of the Bronsted acidity in TS-1 zeolite

As discussed in the above sections, there are Bronsted acidic

sites present in TS-1 zeolite which are caused by a novel defect

site (dsII). It is well known that the strength of the Bronsted

acidity can be well estimated by the parameter of proton affinity

(PA) [42–44]. Zeolite clusters with higher PA are generally

poorer proton donors and correspondingly have lower Bronsted

acidities.

The proton affinity was obtained through the deprotonation

energy (Edep) as shown in the equation below:

Zeo�O�H ! Zeo�O� þHþ (5)

PA ¼Edep¼EðZeo�O�Þ�EðZeo�O�HÞ (6)

where E(Zeo–O–H) and E(Zeo–O�) represent the energies of

the zeolite clusters before and after deprotonation, respectively.

By optimizing the protonated and deprotonated cluster models

of defect site II, the proton affinity of TS-1 zeolite was

calculated with the value of 307.4 kcal mol�1. In the same

way, the proton affinities of defect site I in TS-1, defect site I

and defect site II in silicalite-1 were also computed, with their

exact values equaling 322.2, 340.6 and 324.9 kcal mol�1,

respectively. Accordingly, defect site II in TS-1 zeolite has

the strongest acidity among these four defect sites, which is the

reason that only this defect site facilitates the proton transfer-

ring process towards the probe molecules.

Cations of +III valence state such as B, Al and Fe were often

incorporated into MFI-type zeolites, with the strengths of their

Bronsted acidities well understood [43,45]. Accordingly, it is

feasible to know the Bronsted acidity of TS-1 zeolite through

the comparisons with these three cation-doped zeolites.

Clusters of the same size were selected from these three

cation(III)-doped MFI-type zeolites and optimized at depro-

tonated and protonated states, respectively (see the Al-ZSM-5

clusters as the examples in Fig. 8). The bridging O–H distances

were optimized at 0.969, 0.978, 0.976 A´

for B-, Al- and Fe-

doped MFI-type zeolites, respectively. The proton affinities of

B-, Al- and Fe-doped MFI-type zeolites were calculated to

decrease in the order B-ZSM-5 (303.6 kcal mol�1) > Fe-ZSM-

5 (292.8 kcal mol�1) > Al-ZSM-5 (288.2 kcal mol�1), agree-

ing well with previous theoretical results [43,45]. With TS-1

zeolite also considered, the sequence of proton affinities

follows as TS-1 > B-ZSM-5 > Fe-ZSM-5 > Al-ZSM-5. It

was found that the proton affinity of TS-1 zeolite is even

larger than that of B-ZSM-5, indicating a weaker Bronsted

acidity in TS-1 zeolite than in B-ZSM-5 zeolite. Moreover, it

was found that the difference of the proton affinities between

TS-1 and B-ZSM-5 is equal to 3.8 kcal mol�1, which is slightly

smaller than that between Fe-ZSM-5 and Al-ZSM-5

(4.6 kcal mol�1). Different reaction processes are catalyzed

by the Bronsted acidic sites of different strengths. The TS-1

zeolite is peculiar in that it has a very weak Bronsted acidity.

Accordingly, it may find potential applications in the acid

catalysis, which we believe will be an interesting topic in the

future.

Fig. 8. The H-form (a) cluster of Al-ZSM-5 zeolite and its deprotonated (b) configuration.

G. Yang et al. / Applied Catalysis A: General 337 (2008) 58–6564

5. Conclusions

In the present work, the Bronsted acidities and defect sites in

TS-1 zeolite were investigated with density functional

calculations as well as XRF, XRD, UV–vis and NMR

experiments. Owing to TMP adsorption, the Q3 species

[(SiO)3Si(OH)] in TS-1 zeolite were converted into the Q4-

like species [Si(OSi)4], suggesting the presence of Bronsted

acidity in TS-1 zeolite. Moreover, the amount of Bronsted

acidic sites has direct relationship with the content of

framework Ti species. The Bronsted acidity was caused by a

novel defect site (defect site II, dsII). It was found that the

proton transfer processes of defect site II was much more

facilitated in TS-1 zeolite than in silicalite-1, and accordingly

TS-1 zeolite shows Bronsted acidity whereas silicalite-1 does

not. The theoretical calculations on proton affinities showed

that the Bronsted acidities of four cation-doped MFI-type

zeolites decrease in the order of Al-ZSM-5 > Fe-ZSM-5 > B-

ZSM-5 > TS-1. The unique weak Bronsted acidity in TS-1

zeolite may be an indication of potential applications as acid

catalysts. As to silicalite-1, its acidity is even slightly weaker

than that of defect site I in TS-1 zeolite.

Acknowledgement

This work was supported by National Natural Science

Foundation of PR China (90206036).

References

[1] M. Taramasso, G. Perego, B. Notari, US Patent 4410501, 1983.

[2] B. Notari, Adv. Catal. 41 (1996) 253 (and references therein).

[3] G.M. Lerici, G. Bellussi, U. Romano, J. Catal. 129 (1991) 159.

[4] I.W.C.E. Arends, R.A. Sheldon, M. Wallau, U. Schuchardt, Angew. Chem.

Int. Ed. 36 (1997) 1144.

[5] P.E. Sinclair, G. Sankar, C.R.A. Catlow, J.M. Thomas, T. Maschmeyer, J.

Phys. Chem. B 101 (1997) 4232.

[6] D. Tantanak, M.A. Vincent, I.H. Hillier, Chem. Commun. (1998) 1031.

[7] C.A. Hijar, R.M. Jacubinas, J. Eckert, N.J. Henson, P.J. Hay, K.C. Ott, J.

Phys. Chem. B 104 (2000) 12157.

[8] C. Lamberti, S. Bordiga, A. Zecchina, G. Artioli, G. Marra, SpanoF G., J.

Am. Chem. Soc. 123 (2001) 2204.

[9] H. Munakata, Y. Oumi, A. Miyamoto, J. Phys. Chem. B 105 (2001) 3493.

[10] A. Damin, S. Bordiga, A. Zecchina, C. Lamberti, J. Chem. Phys. 117

(2002) 226.

[11] M. Sasidharan, P. Wu, T. Takashi, J. Catal. 209 (2002) 260.

[12] D. Srinivas, P. Manikandan, S.C. Laha, R. Kumar, P. Ratnasamy, J. Catal.

217 (2003) 160.

[13] J.Q. Zhuang, D. Ma, Z.M. Yan, X.M. Liu, X.W. Han, X.H. Bao, Y.H.

Zhang, X.W. Guo, X.S. Wang, Appl. Catal. A: Gen. 258 (2004) 1.

[14] J.Q. Zhuang, D. Ma, Z.M. Yan, F. Deng, X.M. Liu, X.W. Han, X.H. Bao,

X.W. Liu, X.W. Guo, X.S. Wang, J. Catal. 221 (2004) 670.

[15] J.Q. Zhuang, G. Yang, D. Ma, X.J. Lan, X.M. Liu, X.W. Han, X.H. Bao, U.

Muller, Angew. Chem. Int. Ed. 43 (2004) 6377.

[16] G. Yang, L.J. Zhou, X.C. Liu, X.W. Han, X.H. Bao, Struct. Chem. 18

(2007) 353.

[17] E. Jorda, A. Tuel, R. Teissier, J. Kervennal, J. Catal. 175 (1998) 93.

[18] S. Krijnen, B.L. Mojet, H.C.L. Abbenhuis, J.H.C. van Hooff, R.A. van

Santen, Phys. Chem. Chem. Phys. 1 (1999) 361.

[19] R. Millini, E. Massara, G. Perego, G. Bellussi, J. Catal. 137 (1992) 497.

[20] G. Ricchiardi, A. Damin, S. Bordiga, C. Lamberti, G. Spano, F. Rivetti, A.

Zecchina, J. Am. Chem. Soc. 123 (2001) 11409.

[21] T. Blasco, M.A. Cambor, A. Corma, P. Esteve, J.M. Guil, A. Martinez, J.A.

Perdigon-Melon, S.J. Valencia, J. Phys. Chem. B 102 (1998) 75.

[22] S. Bordiga, A. Damin, F. Bonino, A. Zecchina, G. Spano, F. Rivetti, V.

Bolis, C. Prestipino, C. Lamberti, J. Phys. Chem. B 106 (2002) 9892.

[23] N.U. Zhanpeisov, M. Anpo, J. Am. Chem. Soc. 126 (2004) 9439.

[24] D.H. Wells, W.N. Delgass, K.T. Thomson, J. Am. Chem. Soc. 126 (2004)

2956.

[25] G. Yang, J.Q. Zhuang, D. Ma, X.J. Lan, L.J. Zhou, X.C. Liu, X.W. Han,

X.H. Bao, J. Mol. Struct. (2007), doi:10.1016/j.molstruc.2007.09.005.

[26] B.Y. Zhao, H.J. Pan, J.H. Lunsford, Langmuir 15 (1999) 2761.

[27] F.H. Mueller, N. Rieber, BASF, Patent WO 1998/047845.

[28] F.H. Mueller, N. Rieber, BASF, Patent WO 2001/014251.

[29] W.P. Zhang, D. Ma, X.C. Liu, X.W. Han, X.H. Bao, Chem. Commun.

(1999) 1091.

[30] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R.

Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C.

Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain,

O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.

Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala,

Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B.

Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu,

A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J.

Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez,

M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L.

Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople Gaus-

sian 98, Revision A.9 Gaussian, Inc., Pittsburgh, PA, 1998.

[31] A.D. Becke, Phys. Rev. A 38 (1988) 3098.

[32] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.

[33] T.H.J. Dunning, P.J. Hay, in: H.F. Schaefer, III (Ed.), Modern Theoretical

Chemistry, Plenum Press, New York, 1976, pp. 1–35.

G. Yang et al. / Applied Catalysis A: General 337 (2008) 58–65 65

[34] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270.

[35] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299.

[36] G. Yang, D.H. Zhou, X.C. Liu, X.W. Han, X.H. Bao, J. Mol. Struct. 797

(2006) 131.

[37] G. Yang, J.L. Zhou, X.C. Liu, X.W. Han, X.H. Bao, J. Phys. Chem. B 110

(2006) 22295.

[38] J. Klass, K. Kulawik, G. Schulz-Ekloff, N.I. Jaeger, Stud. Surf. Sci. Catal.

84 (1994) 2261.

[39] J.S. Reddy, R. Kumar, P. Ratnasamy, Appl. Catal. A 58 (1990) L1.

[40] W.O. Parker, Magn. Reson. Chem. 37 (1999) 433.

[41] C. Lamberti, S. Bordiga, D. Arduino, A. Zecchina, F. Geobaldo, G. Spano,

F. Genoni, G. Petrini, A. Carati, F. Villain, G. Vlaic, J. Phys. Chem. B 102

(1998) 6382.

[42] A. Redondo, P.J. Hay, J. Phys. Chem. 97 (1993) 11754.

[43] A. Chatterjee, T. Iwasaki, T. Ebina, A. Miyamoto, Micro. Meso. Mater. 21

(1998) 421.

[44] G. Yang, X.C. Liu, X.W. Han, X.H. Bao, J. Phys. Chem. B 110 (2006)

23388.

[45] S.P. Yuan, J.G. Wang, Y.W. Li, H.J. Jiao, J. Mol. Catal. A: Chem. 106

(2002) 8167.